Retinal rod oute segments (ROS) contain high levels of compounds which may become toxic upon oxidation and damage the retina and other ocular tissues; this process is assumed to take place in diseases like retrolental fibroplasia. The mechanisms which normally prevent this oxidation are not known, and the possible role of pigment epithelium (PE) cells was tested here. PE cells were found to be resistant to ROS in culture at concentrations which were markedly toxic to cells like lymphocytes. Furthermore, PE cells provided a partial protection to lymphocyte cultures affected by ROS. The retinopathy of RCS rats is attributed to deficiency of their PE cells to phagocytose ROS. The possibility that macrophages (Mphi) of these rats are also deficient in their capacity to phagocytose ROS was tested and ruled out: Mphi from RCS rats resembled Mphi from all control rats in their phagocytosis of ROS.